Hi if you read the "terminology" of a Cat D write off its only classed as minor damage
"D - Repairable salvage. Minimal damage sometimes stolen and found after claim has been paid, or cost of repair combined with difficulty obtaining new parts to enable a swift repair."
if you look at this website it explains it a bit better
http://www.wcsauto.com/salvage/categories/inde x.html
I would just sell the car BUT explain that it is a CAt D and be honest. Contact the DVLA they should be able to tell you exactly what was done to it / what happened. God luck